R.G. Neves

Bio: R.G. Neves is an academic researcher from Universidade Federal de Goiás. The author has contributed to research in topics: Sintering & Grain size. The author has an hindex of 4, co-authored 9 publications receiving 41 citations. Previous affiliations of R.G. Neves include Carlos III Health Institute.

Improvement of Ti Processing through Colloidal Techniques

Abstract: The colloid-chemistry control of metallic powders in aqueous slurries is proposed as a way to prepare Ti powders with small particle size for a better pressing behavior through the spray dry process. The chemical-physic behavior of titanium powders with two different particle size distributions dispersed in water has been studied by measuring the zeta potential as a function of pH, and dispersant concentration. The employment of poly-acrylic dispersants allowed the fabrication of stable slurries with solid contents up to 50 vol% that have been sprayed under different conditions to form agglomerates ranging between 50 and 200 µm. Conditions were selected to achieve spherical agglomerates formed by a broad distribution of particle sizes that shown excellent flowability. Agglomerates were pressed in a uniaxial die to measure the compressibility, showing an improvement in pressing behavior with respect to powders with bigger particle size. The sintering behavior is also improved, as values of 96 % of the theoretical density were obtained for compacts sintered in vacuum at 1100 oC for 30 minutes.

Improvement of sintering behaviour of titanium by colloidal techniques

Abstract: The conventional sintering of titanium requires high temperatures to obtain high densities and low porosity, giving rise to microstructures with high grain size, and high interstitial contents, both of which adversely affect the mechanical properties obtained. A novel approach is reported which uses fine (10 µm) spherical Ti powder to improve the sintering behaviour, together with a small percentage of alumina particles (0·5 µm) to restrict grain growth. Colloidal techniques were used to form spherical agglomerates, 50–300 µm in size, of Ti powder with alumina particles dispersed homogenously on the Ti surface. These agglomerates present good compressibility and make it possible to sinter effectively at low temperatures, increasing the relative density and decreasing grain size.

Improvement of TiN nanoparticles EPD inducing steric stabilization in non-aqueous suspensions

Abstract: In this work, the process parameters involved in the electrophoretic deposition (EPD) to obtain thin TiN films was studied and optimised. The stability of commercial TiN nanopowder in isopropyl alcohol adding a cationic polymer (polyethylenimine) as dispersant, with different molecular weights, was investigated to determine the kinetics of the deposition and to reach the most efficient EPD process. Cathodic EPD was performed over electro-polished stainless steel substrates. It was found that the provided dispersion when the PEI with the highest molecular weight was added to the suspension, leads to the best deposition behaviour for short times. New flocculation phenomena were described which affect to the sticking factor, and thus to the evolution of the EPD kinetics. As a result of the designed stabilization system, a reliable and versatile EPD method to produce well consolidated nano-TiN coatings at 1200 °C in vacuum atmosphere was described.

Titanium sintering science: A review of atomic events during densification

Abstract: The sintering densification trajectory for titanium powder is identified in terms of the interaction between mass transport processes and microstructure evolution. During initial heating, as surface oxides dissolve, surface diffusion forms bonds between contacting particles without densification. Grain boundaries form in the bonds due to random crystal orientations at the contacts. Except for mixed powder Kirkendall swelling, subsequent diffusion in these interparticle grain boundaries leads to densification. Most importantly, the alpha-beta transformation provides strain, defects, and interfaces that accelerate densification in the 800–1100 °C temperature range. This is below a typical peak sintering temperature. Final densification involves beta phase volume diffusion and grain boundary diffusion. Densification slows due to grain growth and the loss of grain boundary area. Pores close near 92% density to trap impurities and reaction products inside the closed pores, often limiting sintered density to about 95% of theoretical. High final density requires slow heating or long holds at intermediate temperatures to evaporate impurities prior to pore closure. The master sintering curve is a means to link densification to process parameters without concern over detailing this cascade of transport mechanisms and microstructure changes.

Functionalizing Ti-surfaces through the EPD of hydroxyapatite/nanoY2O3.

Abstract: Ceramic materials for skeletal repair and reconstruction are expanding to a number of different applications. Present research is addressing new compositions and performances to promote osseo-integration through metal coatings. Nanotechnology plays a key role in this research because nanostructures can be introduced into implants to functionalize them and/or to enhance their properties, such as the thermal or mechanical response. In this work, the insertion of Y(2)O(3) nanoparticles into a hydroxyapatite (HA) coating of Ti using colloidal processing technology was developed. The suspensions of HA and Y(2)O(3) nanoparticles were formulated with a focus on zeta potential, particle size distribution, and viscosity for the codeposition of both phases by electrophoresis. The microstructure of the nanocomposite coating was optimized by adjusting the main parameters of the electrophoretic deposition process. A threshold value of the applied electric field for the composite shaping was identified. The results demonstrate that the Y(2)O(3) nanoparticles are homogeneously distributed in the coating and decrease in concentration as the distance from the substrate increases. As a consequence of the presence of the Y(2)O(3), delays in the HA thermal decomposition and the improvement of metal-ceramic joining were observed.

New approach to improve polymer-Mg interface in biodegradable PLA/Mg composites through particle surface modification

Abstract: PLA/Mg composites have been successfully proposed as biodegradable and fully bioabsorbable materials for bone repair. The present work claims the benefits of Mg particles modification throughout the adsorption of surface modifiers in the in vitro biodegradation behavior of PLA/Mg composites. Additionally, the mixture of PLA with Mg particles in a colloidal suspension replaces the thermal-melting extrusion mixing, preventing the drawbacks associated to the thermal degradation of the polymer during processing. To fulfill this requirement it is necessary to improve the interaction particle-liquid medium for the hydrophilic surface of Mg particles and the organic solvent of PLA, which will provide the maximum dispersion of the Mg in the composite. In this sense, surfaces of Mg particles were modified by the adsorption of two different stabilizers, a surfactant as cetyltrimethylammonium bromide (CTAB) and a polyelectrolyte as polyethylenimine (PEI). The colloidal and chemical stability of Mg was studied in terms of Mg2+ dissolution, zeta-potential and rheology. A chemically stable suspension of modified Mg particles was mixed with the PLA solution in THF, and the mixture was used as feedstock to prepare films by tape casting. The characterization of the composites shows that particle surface modification determines Mg corrosion and hence governs the composite biodegradation.